Many surgical procedures, especially those involving cardiac surgery, require that blood be shunted around the surgical site by means of an extracorporeal circuit. For example, during open heart surgery, the beating of the heart must often be temporarily stopped. Of course, while the heart is stopped, it is still necessary to prevent ischemia to the heart muscle which may result in permanent damage, while also providing circulation of blood to the brain and other vital organs. In connection with the need to oxygenate and circulate the blood, it is often necessary to maintain the blood at a particular temperature or to raise or lower the temperature of the blood.
The heart can be protected during open heart surgery using a method known as cold cardioplegia. In that method, a chilled cardioplegia solution is provided to the heart. The cardioplegia solution typically comprises a crystalloid chemical solution which includes potassium, either alone or in combination with other additives. The solution may also be combined with blood obtained from the patient or other suitable donor. Thus, as used herein, the term "cardioplegia solution" is intended to encompass fluids used in an extracorporeal circuit which comprise crystalloid solution, blood, or any combination of crystalloid solution and blood. The use of chilled cardioplegia solution is known to be effective in maintaining the heart in an arrested state, while simultaneously maintaining an appropriate level of oxygen to the heart muscle. Thus, a means must be provided for chilling the cardioplegia solution as well as for returning the solution to physiological temperature.
More recently, a new procedure, referred to as warm continuous blood cardioplegia, has attracted some interest among cardiac surgeons. In this procedure, the cardioplegia solution is not cooled. That notwithstanding, it may still be desirable to maintain the ability to control the temperature of cardioplegia solution in the extracorporeal circuit.
In either method, temperature of the cardioplegia solution can be maintained or controlled using a heat exchanger. It is desirable that the heat exchanger be designed to allow highly efficient heat transfer without causing a substantial pressure drop. It is known in the art to use heat exchangers having a corrugated metal core, for example a corrugated stainless steel core, as the heat transfer barrier. The use of such cores is desirable because the metal offers excellent heat transfer characteristics while acting as a strong barrier between the fluids among which heat is being exchanged. The corrugations serve to increase the total surface area available for heat transfer while still allowing the device to remain relatively compact. Although concurrent and cross-current designs are known in the art, preferred heat exchangers have counter-current designs. In these systems, a first fluid flows along one side of the barrier in one direction and a second fluid flows along the other side of the barrier in a parallel, but opposite, direction.
In the case of a heat exchanger being used to control the temperature of cardioplegia solution, the solution can be chilled or heated as it flows along one side of the heat exchange barrier by water which is flowed in the opposite direction along the other side of the barrier. It is desirable that the side of the heat exchange device which accommodates the cardioplegia solution be designed to minimize the pressure drop between the device inlet and the device outlet. Additionally, the solution side of the heat exchanger is preferably designed to: a) minimize the formation of air bubbles, b) maximize the ability to entrap any air bubbles which may be present, c) eliminate stagnant zones, and d) minimize damage to blood cells that may be present in the solution.
In contrast, since water is typically used as the medium with which the cardioplegia solution exchanges heat, and since the heat exchange water will not be subjected to the limitations of operating within a complex biological circuit, many of the considerations applicable to the cardioplegia solution side of the barrier do not apply on the water side of the barrier.
In the discussion above, heat exchangers have been described for use in systems adapted for handling cardioplegia solution. It should be understood, however, that similar conditions and considerations are applied when using heat exchangers in other medical apparatus. For example, U.S. Pat. No. 5,421,405 (Goodin et al.), the teachings of which are incorporated herein by reference, describes a heat exchanger for use in blood oxygenation systems.
Despite the well developed art of medical heat exchanger design, a need still exists for improved medical heat exchangers. In particular, a need exists for a heat exchanger adapted for use in medical, and particularly cardioplegia, applications in which heat exchange efficiency is further improved, the pressure drop across the device is minimized, formation of gaseous emboli is prevented, and entrapment of gaseous emboli is enhanced.